Respiration depends upon brainstem neuronal circuits that produce the respiratory rhythm and relay it, through the ventrolateral columns, to motor neurons in the spinal cord. This brainstem system produces respiration automatically, i.e., without conscious effort, and is responsive to chemical and mechanical stimuli that signal imbalances in respiratory homeostasis. In addition to this automatic/metabolic respiratory system, there is a voluntary/behavioral system that controls the respiratory muscles during speaking, breath holding, and other voluntary respirabory acts. Recent results from our laboratory showed that the voluntary/behavioral system interacts with respiratory cells that are part of the automatic/metabolic system. Cats were trained to halt inspiration abruptly and to prolong the following expiration when a tone sounded. Recordings of brainstem respiratory neurons showed that their activity patterns were analogous to the behavioral respiratory response, indicating there was interaction of the voluntary/behavioral system with the automatic/metabolic system, but these recordings did not show which cells might produce the shortened inspiration and the prolonged expiration. The long-term objective is to discover the neural mechanisms of voluntary/behavioral respiratory responses. The specific aims are to record and analyze the activity of three groups of respiratory cells, each of which may have a role in either the inhibition of inspiration or the switching of the respiratory cycle from one phase to the other--necessary acts in the performance of the behavioral respiratory response. These three groups are: 1) the expiratory cells in the region of the retrofacial nucleus, 2) the inspiratory cells of the ventrolateral nucleus of tractus solitarius, and 3) the phase-spanning cells of the pontine pneumotaxic center. For these experiments, cats will be trained to stop inspiration and prolong expiration when a tone sounds. Single neurons within the three specified groups will be recorded while the animals perform these responses. Cells mediating the response should be activated during the task. Our goal is to identify the cells that inhibit inspiration and prolong expiration when the animals produce these behavioral responses.